1. Field
The present invention relates to a wavelength selective optical switch which may be applied to a wavelength selective optical switch using MEMS (Micro Electro Mechanical Systems) mirrors.
2. Description of the Related Art
In a Wavelength Division Multiplexing (“WDM”) optical communication system, an optical switch that performs a wavelength selection is used to switch optical paths.
As one mode of such an optical switch (wavelength selective optical switch) that performs an optical signal selection based on a wavelength, a configuration applying MEMS (Micro Electro Mechanical Systems) technology using mirrors is known.
FIGS. 1A and 1B are respectively a top view and a side view of a typical wavelength selective optical switch 1. The wavelength selective switch 1 has a first optical element 100, a wavelength dividing unit 101, a second optical element 102, and an optical signal processing unit 103.
As illustrated in FIG. 1B, the first optical element 100 has an optical fiber having a plurality of input ports IN and a plurality of output ports OUT, and a collimating lens (not shown).
The wavelength dividing unit 101 is, for instance, a diffraction grating that divides WDM signal lights (wavelengths λ1 to λm) entering from the input ports IN of the first optical element 100, into optical signals of m different wavelengths.
As illustrated in FIG. 1A, the second optical element 102 inputs the divided lights from the wavelength dividing unit 101 and guides the lights to the optical signal processing unit 103.
The optical signal processing unit 103 is, for instance, folding type mirrors of an MEMS array 10A, which includes m MEMS mirrors 10 arranged in an array.
FIG. 2 is a diagram illustrating one of the MEMS mirrors 10 in the MEMS array 10A. The MEMS mirror 10 is driven to change its angle around an X-axis and a Y-axis, by which the MEMS mirror 10 is supported. For example, the MEMS mirror 10 can be driven to change its angle around the X-axis and the Y-axis, by generating electrostatic attractive force by a voltage applied to an electrode which is disposed on the rear side of the MEMS mirror 10.
In the configuration illustrated in FIG. 1A, by changing the angle of one of the MEMS mirrors 10A, selects a direction of the folding mirror in which divided lights entering the MEMS mirror from the wavelength dividing unit 101 are to be sent, and further selects an output port to which reflected lights returning from the folding mirror are to be output.
As described above, in the MEMS array 10A of the optical signal processing unit 103, by changing the angles around the X-axis of the MEMS mirrors 10 disposed in an array, reflected lights with corresponding wavelengths can be coupled from an arbitrary input port IN to an arbitrary output port OUT.
Here, when there are only one input port IN and there are plurality of output ports OUT, the switch operates as a DROP-type wavelength selective optical switch that couples signal lights with an arbitrary wavelength from the one input port IN to an arbitrary output port OUT.
On the contrary, when there is a plurality of input ports IN and one output port OUT, the switch operates as an ADD-type wavelength selective optical switch that couples signal lights with an arbitrary wavelength from an arbitrary input port IN to the one output port OUT.
At this time, such setting can be performed that by further slightly rotating a MEMS mirror around the X-axis or the Y-axis, signal lights with an arbitrary attenuation are coupled from the input port IN to an arbitrary output port OUT.
That is, lights reflected from the optical signal processing unit 103 are collimated by the second optical element 102 and the collimated reflected lights with all wavelengths follow the same path via the wavelength dividing unit 101. Then, the lights are guided to the output ports OUT of the first optical element 100. At that time, the reflected lights are controlled on a wavelength-by-wavelength basis by controlling the angles around the X-axis or the Y-axis of the MEMS mirrors 10 of the optical signal processing unit 103, and are coupled to a fiber end of an arbitrary output port OUT, with attenuations given by the angles around the X-axis or the Y-axis of the MEMS mirrors 10.
At this time, when, for example, there are only one input port and one output port, only attenuations for respective wavelengths are set by the angles around the X-axis or the Y-axis of the MEMS mirrors 10 of the MEMS array 10A.
FIG. 3 is a graph illustrating attenuation relative to an angle of a MEMS mirror 10 (FIG. 1A). A third optical element is an optical element (not illustrated) between the MEMS mirror 10 in the optical signal processing unit 103 and the second optical element 102. The graph illustrates, as an example, the attenuation relative to F×θ, where F is a focal length of a third optical element and θ is an angle of the MEMS mirror 10 and the third optical element.
Additionally, aside from a method for controlling attenuations by changing the angles of the MEMS mirrors 10, the optical signal processing unit 103 may be configured as an optical signal processing unit illustrated in FIG. 4. In FIG. 4, the optical signal processing unit has a MEMS array 10A, which selects a port in the third optical element, and a liquid crystal array 11A. The MEMS array 10A includes a plurality of MEMS mirrors 10. The liquid crystal array 11A includes a plurality of liquid crystal units and each of the plurality of liquid crystal units is provided for the respective MEMS mirrors 10 and gives attenuation. In this configuration, each MEMS mirror 10 is configured to be able to rotate only around the X-axis to select a port.
FIG. 5 is a diagram illustrating the attenuation (vertical axis) relative to the applied voltage (horizontal axis) of a liquid crystal unit. By controlling applied voltages to the liquid crystal units which are provided for the respective MEMS mirrors 10 of the MEMS array 10A, attenuations can be controlled.
FIGS. 6A and 6B are diagrams illustrating a wavelength selective optical switch 6, where an output monitoring on a wavelength-by-wavelength basis is further added to an ADD-type wavelength selective optical switch.
The output monitoring is configured such that some of WDM signal lights at an output port OUT are divided by a coupler 100A and the divided lights are allowed to enter a wavelength monitoring unit 20.
As illustrated in FIGS. 6A and 6B, the wavelength monitoring unit 20 includes a third optical element 200, a wavelength dividing unit 201 including a diffraction grating, and a PD (photodetector unit) array 203. Elements of the PD array 203 respectively correspond to wavelengths of WDM signal lights and thus can detect light intensities for the respective wavelengths of the WDM signal lights.
Information detected by the PD array 203 is used, if necessary, for feedback control for the purpose of angle correction of a MEMS array 10A, etc.
A configuration that has an optical signal processor which performs a process on lights input to an input port, according to a wavelength thereof and outputs the processed lights from an output port, and a monitoring unit which extracts and receives some of the lights processed according to the wavelength thereof, when the lights are output from the output port, to monitor the received lights is described in, for example, Japanese Patent Application Laid-Open No. 2005-301123.
In the above-described configuration of a wavelength selective optical switch, attenuations are set according to change in angles of the MEMS mirrors 10 composing the MEMS array 10A.
The angle of a MEMS mirror 10 is normally changed by a voltage to be applied thereto and attenuation is given by the angle, as illustrated in FIG. 3. Hence, there is a need to install in an apparatus in advance voltages to be applied to the MEMS mirror 10 and attenuations in the form of a numerical value table and obtain, upon actual use, a voltage value required to have required attenuation from the table.
Here, a required angle of the MEMS mirror 10 is very small and attenuation relative to a voltage to be applied has low accuracy and furthermore varies with, for example, a temperature. Also, as described in FIG. 2, when the MEMS mirror 10 is driven to change its angle around the X-axis and the Y-axis by generating electrostatic attractive force by a voltage applied to an electrode which is disposed on the rear side of the MEMS mirror 10, there is a problem specific to the MEMS mirror 10 that charges are accumulated in a mirror portion and accordingly the voltage-angle characteristics gradually change.
In the case of an ADD-type wavelength selective optical switch, as illustrated in FIGS. 6A and 6B, in order to obtain required attenuation, the MEMS mirrors 10 are feedback-controlled in a manner such that some of lights at the output port OUT are divided by the coupler 100A and the signal intensities of respective wavelength components are monitored by an array-type detector unit including the PD array 203.
However, for example, upon activation of an apparatus or when optical signals are cut off for some kind of reason such as due to failure, e.g., cut-off fiber, there is no optical signal input to the wavelength selective optical switch and thus lights at the output port OUT cannot be monitored.
For example, a situation where no optical signals arrive at all for some kind of reason will be considered. In such a case, the switch has to set a fixed attenuation using a preset voltage/attenuation table and then wait.
In this state, for example, when the attenuation is actually greater than a required set value, even if optical signals recover, since the setting for attenuating more than necessary is performed earlier, there is a problem that the wavelength monitoring unit 20 in FIGS. 6A and 6B may determine that sufficient optical signal intensity has not arrived.
On the other hand, when the attenuation is actually smaller than a required set value, even if optical signals recover, since the signals are not sufficiently attenuated, signals with excess intensity are output from the output port OUT. Accordingly, there is a concern that influence such as physical destruction may be exerted on an optical transmission apparatus at a subsequent stage.